1. Field of the Invention
This invention relates to a winding shaft for a belt roll-up device for winding and unwinding a vehicle safety belt, which can rotate in the belt roll-up housing for winding and unwinding the belt, where a locking element connected to the winding shaft comes into engagement with the blocking element fastened to the housing, by a force in the direction of the belt pull, exerted upon the winding shaft.
2. Description of the Invention
Belt roll-up devices of the above-mentioned type are generally known. In the various known designs, a locking element firmly connected to the winding shaft is in engagement with a monitoring device which brings the locking element into engagement with the blocking body fixed to the housing if the permissible angular acceleration of the winding shaft is exceeded, so that further unwinding of the belt is prevented so as to hold back the vehicle passenger who has fastened the safety belt. The monitoring device does not respond if the belt is unwound from the winding shaft slowly and without jerking, as usually takes place when the safety belt is applied so that the locking element and the blocking body remain out of engagement and the winding shaft can rotate freely in the belt roll-up housing.
The winding shaft, which is held centered by springs in the belt roll-up housing, rotates if the belt is unwound or rolled up slowly without jerking. The winding shaft is firmly connected to at least one locking element, which can rotate freely in a blocking body fixed relative to the housing, if it is in the centered position. If the safety belt is to be prevented from unwinding when the minimum value of the vehicle deceleration or the belt acceleration is exceeded, then the winding shaft is made to relocate itself from the centered position in the belt roll-up housing against the spring pre-tension acting on the bearing bushings of the winding shaft by a sensor if the minimum value of the vehicle deceleration is exceeded, or by the force which occurs in the belt when the minimum value of the belt acceleration in the belt strap is exceeded, so that the locking elements of the winding shaft come into engagement with the blocking bodies of the belt roll-up housing. The locking elements and the associated blocking bodies are designed so that they can take up without destruction the maximum belt load of 1500 kg required by law in the event of a blocking intervention. Also the winding shaft is designed so that it can withstand without damage the belt force of the same magnitude acting tangentially on the remaining part of the belt which is still wound up. Besides this requirement as to the strength of individual parts of a belt roll-up device, there is a further requirement with respect to the strength of the winding shaft. According to ECE R 16 (Economy Committee of Europe, Rule 16), which also applies to the Federal Republic of Germany, the winding shaft with the safety belt fastened thereto must likewise be able to stand a load of 1500 kg acting on the belt strap with the belt completely unwound, without damage being inflicted thereby on the winding shaft or the belt fastening. Even though such a condition will occur neither in normal operation nor in the event of an accident, it must nevertheless be fulfilled for the belt roll-up device to be approved generally. To meet this requirement, an elaborate and cost-intensive design of the winding shaft has heretofore been necessary. Depending on the design of the fastening of the belt to the winding shaft, bursting or wedging forces occur at the winding shaft under such a load. These loads on the winding shaft are substantially greater than those which occur tangentially in the belt pull direction with the belt partially rolled up, due to the force of likewise 1500 kg occurring at the belt. A winding shaft is known, for instance, in which there is in the direction of the diameter, a slot of the belt width, which is enlarged at one end. The end of the belt pushed through this slot is folded over and sewn to form a loop. A pin is inserted into this loop. The belt loop with the pin then rests in the enlargement, so that on the one hand, the circular surface of the winding shaft is preserved and, on the other hand, the belt end pushed through the slot cannot slide out under the tensional load. For reasons of strength and dimensional tolerances, this winding shaft is made of diecast zinc. This entails substantial material, manufacturing and assembly costs in the production as well as in the installation of the winding shaft.